3,997 research outputs found
Doctor of Philosophy
dissertationAmplitude modulation (AM) detection measures a listener's sensitivity to temporal envelope fluctuations. AM signals are ecologically relevant because the amplitude of speech fluctuates over time. The post-cochlear representation of AM may be influenced by processes that occur in the cochlea, where signals are subject to cochlear compression and adaptive mechanisms that modulate the cochlear response such as the medial olivocochlear (MOC) reflex. Specifically, cochlear compression may reduce the difference between high-intensity peaks and low-intensity valleys (i.e., effective modulation depth) of AM. Furthermore, gain reduction of the cochlear amplifier via the MOC reflex is hypothesized to decompress the cochlear input-output function and thus improve the AM effective modulation depth at moderate levels. To test these hypotheses, AM detection was measured for a narrow-band, high-frequency carrier (5000 Hz) for conditions that do or do not elicit the MOC reflex. These conditions take advantage of the sluggish onset of the reflex, which exhibits an onset delay (?25 ms) upon stimulation. Specifically, AM detection was measured as a function of level for a 50 ms carrier in the presence and absence of a long ipsilateral notched-noise precursor. A longer carrier (500 ms) without a precursor was also included. For no-precursor condition, AM detection thresholds at moderate carrier levels are poorer compared to low and high levels, consistent with a reduced effective modulation depth due to cochlear compression. In the precursor condition, AM thresholds improved monotonically with carrier level, with the largest improvements seen at moderate levels. This improvement is consistent with decompression of the cochlear input-output function via the MOC reflex. For 500 ms carriers, AM detection thresholds improved by a constant (across all carrier levels) relative to AM thresholds with a precursor, consistent with the longer carrier providing more "looks" to detect the AM signal. In a second experiment, AM thresholds were measured as a function of modulation frequency to examine whether the effects of the precursor depend on the modulation frequency. The results showed that the improvement in AM detection with compared to without a precursor is limited to low modulation frequencies (<60Hz). The experiment in Chapter 3 was designed to examine the effects of cochlear compression on the inherent fluctuations of narrow-band noise carriers. To test this, AM detection was measured for short and long, high- and low-fluctuating noise carriers as a function of carrier level. The results showed that AM thresholds for short, low-fluctuating noise carriers worsened as carrier level increased from low to mid carrier levels and then improved with further increases in carrier level, as found in the previous experiment. This is consistent with greater cochlear compression at moderate levels. For high-fluctuating carriers, AM thresholds were roughly constant across carrier levels. For high-fluctuating carriers, low-level linear and mid-level compressive cochlear response growth may have resulted in constant envelope signal-to-noise ratios, due to the cochlear response growth equally affecting the target modulation and inherent carrier fluctuations. Thus, AM detection for high-fluctuating carriers is constant as a function of carrier level
Photothermal effects in ultra-precisely stabilized tunable microcavities
We study the mechanical stability of a tunable high-finesse microcavity under
ambient conditions and investigate light-induced effects that can both suppress
and excite mechanical fluctuations. As an enabling step, we demonstrate the
ultra-precise electronic stabilization of a microcavity. We then show that
photothermal mirror expansion can provide high-bandwidth feedback and improve
cavity stability by almost two orders of magnitude. At high intracavity power,
we observe self-oscillations of mechanical resonances of the cavity. We explain
the observations by a dynamic photothermal instability, leading to parametric
driving of mechanical motion. For an optimized combination of electronic and
photothermal stabilization, we achieve a feedback bandwidth of kHz and a
noise level of m rms
Time-resolved magnetic sensing with electronic spins in diamond
Quantum probes can measure time-varying fields with high sensitivity and
spatial resolution, enabling the study of biological, material, and physical
phenomena at the nanometer scale. In particular, nitrogen-vacancy centers in
diamond have recently emerged as promising sensors of magnetic and electric
fields. Although coherent control techniques have measured the amplitude of
constant or oscillating fields, these techniques are not suitable for measuring
time-varying fields with unknown dynamics. Here we introduce a coherent
acquisition method to accurately reconstruct the temporal profile of
time-varying fields using Walsh sequences. These decoupling sequences act as
digital filters that efficiently extract spectral coefficients while
suppressing decoherence, thus providing improved sensitivity over existing
strategies. We experimentally reconstruct the magnetic field radiated by a
physical model of a neuron using a single electronic spin in diamond and
discuss practical applications. These results will be useful to implement
time-resolved magnetic sensing with quantum probes at the nanometer scale.Comment: 8+12 page
Modulation masking produced by second-order modulators
Recent studies suggest that an auditory nonlinearity converts second-order sinusoidal amplitude modulation (SAM) (i.e., modulation of SAM depth) into a first-order SAM component, which contributes to the perception of second-order SAM. However, conversion may also occur in other ways such as cochlear filtering. The present experiments explored the source of the first-order SAM component by investigating the ability to detect a 5-Hz, first-order SAM probe in the presence of a second-order SAM masker beating at the probe frequency. Detection performance was measured as a function of masker-carrier modulation frequency, phase relationship between the probe and masker modulator, and probe modulation depth. In experiment 1, the carrier was a 5-kHz sinusoid presented either alone or within a notched-noise masker in order to restrict off-frequency listening. In experiment 2, the carrier was a white noise. The data obtained in both carrier conditions are consistent with the existence of a modulation distortion component. However, the phase yielding poorest detection performance varied across experimental conditions between 0° and 180°, confirming that, in addition to nonlinear mechanisms, cochlear filtering and off-frequency listening play a role in second-order SAM perception. The estimated magnitude of the modulation distortion component ranges from 5%-12%
Eulerian-Lagrangian method for simulation of cloud cavitation
We present a coupled Eulerian-Lagrangian method to simulate cloud cavitation
in a compressible liquid. The method is designed to capture the strong,
volumetric oscillations of each bubble and the bubble-scattered acoustics. The
dynamics of the bubbly mixture is formulated using volume-averaged equations of
motion. The continuous phase is discretized on an Eulerian grid and integrated
using a high-order, finite-volume weighted essentially non-oscillatory (WENO)
scheme, while the gas phase is modeled as spherical, Lagrangian point-bubbles
at the sub-grid scale, each of whose radial evolution is tracked by solving the
Keller-Miksis equation. The volume of bubbles is mapped onto the Eulerian grid
as the void fraction by using a regularization (smearing) kernel. In the most
general case, where the bubble distribution is arbitrary, three-dimensional
Cartesian grids are used for spatial discretization. In order to reduce the
computational cost for problems possessing translational or rotational
homogeneities, we spatially average the governing equations along the direction
of symmetry and discretize the continuous phase on two-dimensional or
axi-symmetric grids, respectively. We specify a regularization kernel that maps
the three-dimensional distribution of bubbles onto the field of an averaged
two-dimensional or axi-symmetric void fraction. A closure is developed to model
the pressure fluctuations at the sub-grid scale as synthetic noise. For the
examples considered here, modeling the sub-grid pressure fluctuations as white
noise agrees a priori with computed distributions from three-dimensional
simulations, and suffices, a posteriori, to accurately reproduce the statistics
of the bubble dynamics. The numerical method and its verification are described
by considering test cases of the dynamics of a single bubble and cloud
cavitaiton induced by ultrasound fields.Comment: 28 pages, 16 figure
Assessment of digital image correlation measurement errors: methodology and results
Optical full-field measurement methods such as Digital Image Correlation (DIC) are increasingly used in the field of experimental mechanics, but they still suffer from a lack of information about their metrological performances. To assess the performance of DIC techniques and give some practical rules for users, a collaborative work has been carried out by the Workgroup “Metrology” of the French CNRS research network 2519 “MCIMS (Mesures de Champs et Identification en Mécanique des Solides / Full-field measurement and identification in solid mechanics, http://www.ifma.fr/lami/gdr2519)”. A methodology is proposed to assess the metrological performances of the image processing algorithms that constitute their main component, the knowledge of which being required for a global assessment of the whole measurement system. The study is based on displacement error assessment from synthetic speckle images. Series of synthetic reference and deformed images with random patterns have been generated, assuming a sinusoidal displacement field with various frequencies and amplitudes. Displacements are evaluated by several DIC packages based on various formulations and used in the French community. Evaluated displacements are compared with the exact imposed values and errors are statistically analyzed. Results show general trends rather independent of the implementations but strongly correlated with the assumptions of the underlying algorithms. Various error regimes are identified, for which the dependence of the uncertainty with the parameters of the algorithms, such as subset size, gray level interpolation or shape functions, is discussed
Characterization of causes of signal phase and frequency instability Final report
Characteristic instabilities in phase and frequency errors of reference oscillator
Interaural Correlation Fails to Account for Detection in a Classic Binaural Task: Dynamic ITDs Dominate N0Sπ Detection
Binaural signal detection in an NoSπ task relies on interaural disparities introduced by adding an antiphasic signal to diotic noise. What metric of interaural disparity best predicts performance? Some models use interaural correlation; others differentiate between dynamic interaural time differences (ITDs) and interaural level differences (ILDs) of the effective stimulus. To examine the relative contributions of ITDs and ILDs in binaural detection, we developed a novel signal processing technique that selectively degrades different aspects (potential cues) of binaural stimuli (e.g., only ITDs are scrambled). Degrading a particular cue will affect performance only if that cue is relevant to the binaural processing underlying detection. This selective scrambling technique was applied to the stimuli of a classic N0Sπ task in which the listener had to detect an antiphasic 500-Hz signal in the presence of a diotic wideband noise masker. Data obtained from five listeners showed that (1) selective scrambling of ILDs had little effect on binaural detection, (2) selective scrambling of ITDs significantly degraded detection, and (3) combined scrambling of ILDs and ITDs had the same effect as exclusive scrambling of ITDs. Regarding the question which stimulus properties determine detection, we conclude that for this binaural task (1) dynamic ITDs dominate detection performance, (2) ILDs are largely irrelevant, and (3) interaural correlation of the stimulus is a poor predictor of detection. Two simple stimulus-based models that each reproduce all binaural aspects of the data quite well are described: (1) a single-parameter detection model using ITD variance as detection criterion and (2) a compressive transformation followed by a crosscorrelation analysis. The success of both of these contrasting models shows that our data alone cannot reveal the mechanisms underlying the dominance of ITD cues. The physiological implications of our findings are discussed
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